Move Compiler <-> OpaqueClosure interface code to Compiler (JuliaLang#56576)

After the excision, it is no longer permissable for Base to have
`Compiler` data structures in arguments of methods it defines. To comply
with this restriction, move the functions for creating OpaqueClosures
from IRCode to `Compiler`.

Author: Keno

src/Compiler.jl

@@ -179,6 +179,7 @@ include("optimize.jl")
 
 include("bootstrap.jl")
 include("reflection_interface.jl")
+include("opaque_closure.jl")
 
 module IRShow end
 if !isdefined(Base, :end_base_include)

src/opaque_closure.jl

@@ -0,0 +1,56 @@
+# This file is a part of Julia. License is MIT: https://julialang.org/license
+
+function compute_ir_rettype(ir::IRCode)
+    rt = Union{}
+    for i = 1:length(ir.stmts)
+        stmt = ir[SSAValue(i)][:stmt]
+        if isa(stmt, Core.ReturnNode) && isdefined(stmt, :val)
+            rt = Compiler.tmerge(Compiler.argextype(stmt.val, ir), rt)
+        end
+    end
+    return Compiler.widenconst(rt)
+end
+
+function compute_oc_signature(ir::IRCode, nargs::Int, isva::Bool)
+    argtypes = Vector{Any}(undef, nargs)
+    for i = 1:nargs
+        argtypes[i] = Compiler.widenconst(ir.argtypes[i+1])
+    end
+    if isva
+        lastarg = pop!(argtypes)
+        if lastarg <: Tuple
+            append!(argtypes, lastarg.parameters)
+        else
+            push!(argtypes, Vararg{Any})
+        end
+    end
+    return Tuple{argtypes...}
+end
+
+function Core.OpaqueClosure(ir::IRCode, @nospecialize env...;
+                            isva::Bool = false,
+                            slotnames::Union{Nothing,Vector{Symbol}}=nothing,
+                            kwargs...)
+    # NOTE: we need ir.argtypes[1] == typeof(env)
+    ir = Core.Compiler.copy(ir)
+    # if the user didn't specify a definition MethodInstance or filename Symbol to use for the debuginfo, set a filename now
+    ir.debuginfo.def === nothing && (ir.debuginfo.def = :var"generated IR for OpaqueClosure")
+    nargtypes = length(ir.argtypes)
+    nargs = nargtypes-1
+    sig = compute_oc_signature(ir, nargs, isva)
+    rt = compute_ir_rettype(ir)
+    src = ccall(:jl_new_code_info_uninit, Ref{CodeInfo}, ())
+    if slotnames === nothing
+        src.slotnames = fill(:none, nargtypes)
+    else
+        length(slotnames) == nargtypes || error("mismatched `argtypes` and `slotnames`")
+        src.slotnames = slotnames
+    end
+    src.slotflags = fill(zero(UInt8), nargtypes)
+    src.slottypes = copy(ir.argtypes)
+    src.isva = isva
+    src.nargs = UInt(nargtypes)
+    src = ir_to_codeinf!(src, ir)
+    src.rettype = rt
+    return Base.Experimental.generate_opaque_closure(sig, Union{}, rt, src, nargs, isva, env...; kwargs...)
+end